Asphalt roadways are constructed from mixtures of asphalt cement (bitumen) and aggregate. Over time, the road suffers damage due to traffic and/or environmental factors and some of this damage can be related to the properties of the asphalt cement. For example, rutting from heavy vehicle traffic is often a result from too soft asphalt binder at high temperatures and cracking can be a result of too high asphalt binder stiffness at low temperatures. Due to the increase in road traffic, the high costs of repair and replacement, and the decline in asphalt cement quality, the modification of the high and low temperature properties of asphalt cement (bitumen) by means of additives is often necessary to improve the durability of the road.
In the following, the terms asphalt and bitumen are used to describe natural or petroleum-derived bitumen including the well-known penetration grade bitumen, blown or oxidized grades and polymer-modified bitumen, for example, modified with styrene-butadiene polymers or ethylene vinyl acetate polymers or ground tire rubber. The terms asphalt and bitumen are also considered to include asphalts as described above which may be modified with chemical additives such as adhesion promoters, polyphosphoric acid, or sulfur.
Road construction and repair materials are most commonly prepared by heating asphalt cement to a fluid state and mixing directly with heated aggregate forming “hot mix” which is placed onto the roadway and compacted while still hot. Alternatively the asphalt cement may be fluidified by dilution with solvents, or emulsified with water and emulsifier and mixed with aggregates to provide materials that can be used at lower temperatures. In other techniques fluidified asphalt is sprayed onto the road surface.
Harder, higher viscosity asphalts mostly find application in the roofing industry, but also in some special paving applications such as bond coats. High viscosity asphalts can be prepared by air-blowing (“oxidation”) of soft asphalts, by solvent deasphalting processes at the refinery or by the use of natural asphalts like Gilsonite®. Changes in refinery processes have limited the availability and raised the cost of hard asphalts in some regions.
Asphalt cement can be characterized by its consistency at different temperatures and viscosity measurements are used to classify asphalts into different grades suitable for different applications and climates. Several different rheological tests or combinations of tests may be used to classify asphalt cement. Penetration value is used to measure hardness—the harder the material, the smaller the penetration value. Softening point is the temperature at which the asphalt cement begins to flow freely, and is particularly used for air-blown asphalts.
For hot mix paving applications, the Strategic Highway Research Program (SHRP) has developed asphalt binder specifications, including high temperature tests and low temperature tests, performed on asphalt cement before and after accelerated ageing. The high temperature tests determine the viscoelastic characteristics of the asphalt to control pavement rutting. The low temperature tests determine the cold flow properties of the asphalt to control low temperature cracking. The oven aging tests predict aging characteristics to estimate binder properties after extended periods of time on the road. SHRP binder grades have the form PG XX-YY, where XX is the maximum pavement temperature in degrees Celsius and −YY is the minimum pavement temperature in degrees Celsius. For instance, a binder grade of PG 64-28 means that the binder gives acceptable performance in the temperature range from −28° C. to 64° C. at normal traffic loads. As such, this system defines climate-related optimal working conditions of asphalt binder through specifications for high and low temperature properties of asphalt cement that correlate to road performance. An increase in the upper performance grade of asphalt helps a road resist rutting, while an improved lower performance grade helps a road resist thermal cracking.
The viscosity of the asphalt cement at high temperatures needed for the production and paving of the hot mix is another important parameter measured in the PG grading system. More viscous binders require higher processing, paving and compaction temperatures.
Conventional bitumen compositions frequently cannot meet all of the requirements of a particular specification simultaneously and, if these specifications are not met, damage to the resulting road can occur, including permanent deformation, thermally induced cracking and flexural fatigue. This damage greatly reduces the effective life of paved roads.
To meet performance needs with asphalt cement, performance modifying additives are used. Asphalt cement is modified using polymer modifiers such as styrene-butadiene-styrene (SBS), polyethylene (PE) or other polymers, using wax-type compounds such as montan wax, Fischer-Tropsch wax, amide waxes or using inorganic modifiers such as hydrated carbonate rock.
Polymer modifiers, such as SBS, can improve both the high temperature and low temperature PG grades. However, in addition to being quite difficult to manufacture, involving high shear mixing, polymer-modified binders also have high viscosity and require higher mix and paving temperatures.
High melting point waxes, such as Fischer-Tropsch, paraffin and montan waxes, also stiffen asphalt cement. They are more easily incorporated into asphalt than polymer modifiers, and often reduce the high temperature viscosity and thus allow lower asphalt production and paving temperatures. However, these additives also reduce low temperature flexibility (raise the lower PG grade) and may lead to cracking during cold weather.
High melting point amide waxes can be formed by the reaction of long-chain fatty acids and alkylene diamines, or reaction of long-chain fatty amines and alkylene diacids. These fatty amide waxes, such as ethylenediamine bisstearamide (EBS), have a similar effect in asphalt cement as the petroleum derived waxes but, depending on the source of the fatty acids, offer the potential to have the advantage of coming from primarily renewable raw materials.
Like some of the petroleum waxes, at hot mix production and paving temperatures addition of EBS into asphalt cement reduces the viscosity of the resulting modified asphalt binder. This allows a reduction in mix and paving temperatures compared to unmodified asphalt cement, leading to a consequent reduction in the production of fumes and vapor.
As the mixture cools, the viscosity increases above that of the unmodified asphalt cement and the resulting asphalt-aggregate pavement can sustain heavy loads. Amide waxes can thus be used to suppress rut formation even in hot climates where rutting is common, increasing the life of the asphalt pavement. This has an additional advantage of allowing the use of softer asphalt cement.
A disadvantage of traditional amide waxes is that in common with the petroleum and coal derived waxes, the low temperature flexibility of the modified asphalt binders is decreased compared to unmodified or polymer-modified asphalt binder, reducing their cold weather capability. In terms of the PG grading system described above, the lower PG grade is increased.
Methods used to combat the decrease in low temperature flexibility of paraffin waxes include addition of a vegetable wax, based on modified palm wax esterified with stearic acid, to Fischer-Tropsch wax), and incorporation of isomerized Fischer-Tropsch paraffin waxes.
Ethylene bisstearamide is an example of an amide wax that has been used widely in asphaltic pavements. Use of ethylene bisstearamide to reduce the viscosity of asphalt and increase softening point has been known at least since 1974. Ethylene bisstearamide, used in conjunction with animal wax such as Shellac, has been reported to reduce the temperature needed for placement of asphalt pavement. The product may also be used other asphalt applications outside of hot mix including sprayed asphalt applications, roofing and adhesives.
Methods used to combat the decrease in low temperature flexibility of EBS amide waxes include increasing the amount of the shorter alkyl chain palmitic acid vs. the longer chain stearic acid, especially with incorporation of 1 part in 20 of a dicarboxylic acid, which was found to improve the low temperature properties.
It is therefore an object of the present invention to provide a modifier for asphalt cement which improves rutting and processing characteristics of asphalt pavement without compromising the low-temperature properties of the asphalt cement. Such modified binders would have utility in applications including a compaction aid and performance grade booster for hot mix asphalt, a viscosity reducer for warm mix asphalt, a process aid for asphalt mastics, and as a binder stiffener for trackless tacks, primes and fog seals (either hot applied or using emulsion) or roofing applications. Thin surface treatments such as chipseals, slurry seals, microsurfacing and the like may also benefit from the properties imparted to the asphalt cement of lower temperature susceptibility combined with good low temperature properties.
While most of the discussion has centered on hot and warm mix paving applications, asphalt binders incorporating the present invention would also have utility in emulsions and in non-paving applications. Some examples of non-paving applications are in protective coatings for metal articles, binders for construction boards, waterproofing compositions, asphalt sealcoats for parking lots and driveways, binders for fuel pellets and briquettes, and in roofing materials.
Another object of the invention is to provide an additive which can be obtained from primarily renewable sources.
A third object is to provide an additive which increases the softening point and decreases the penetration value of asphalt binder.